As is commonly known, a large excitation inrush current flows when excitation is performed by supplying power under no load in the state where a magnetic flux remains about an iron core of a transformer. The excitation inrush current is several times greater than the rated load current of the transformer. When such a large excitation inrush current flows, a system voltage fluctuates. If the voltage fluctuation is great, the change influences consumers.
According to a known method for suppressing an excitation inrush current, when a transformer is disconnected from the power supplies, a residual magnetic flux is calculated from a voltage signal on the primary side of a transformer, and a breaker of a reference phase is closed at the timing when a prospective magnetic flux of the reference phase and the residual magnetic flux correspond to each other. Thereafter, the remaining two breakers are closed by causing a delay to minimize a difference between a prospective magnetic flux of another phase and the residual magnetic flux.
However, in a system configuration such as 1½ (one and half) bus configuration which enables a transformer to connect to power supplies by two breakers, which of the two breakers disconnects the transformer from the power supply or connects the transformer to the power supply cannot be known beforehand. Therefore, there is a difficulty in closing the two breakers in phases which suppress an excitation inrush current.
There are relevant prior-art literatures as follows.    Patent Document 1: Jpn. Pat. Appln. KOKAI Publication No. 2009-99347    Patent Document 2: Jpn. Pat. Appln. KOKAI Publication No. 2010-4686    Non-Patent Document 1: John H. Brunke et al., “Elimination of Transformer Inrush Currents by Controlled Switching—Part I: Theoretical Considerations”, IEEE Transactions on Power Delivery, IEEE, April 2001, Vol. 16, No. 2, pp. 276 to 280
Under the circumstances as described above, there is a demand for providing an excitation inrush-current suppression system capable of suppressing an excitation inrush current to be generated when a transformer is connected to power supplies in a system configuration which enables the transformer to connect to the power supplies through two breakers.